This research was aimed at finding the effectiveness of Group Investigation in teaching writing Aannouncement text on first grade students at SMK Karya Teknologi Jatilawang in academic year 2015/2016. This research used quasi experimental design. The participants of this research were the first grade students at SMK Karya Teknologi Jatilawang. They were X MP 3 class and X MP 4 class, which X MP 4 was the experimental class and X MP 3 was the control class. To know the efect of the treatments, pre-test and post-test was done. The test was in the form of structured essay test. The data of the test was analyzed using statistical analysis of the mean and t-test. The mean score of the experimental class was 53,77 for pre-test and 73,44 for post-test, which was there was an improvement as many as 19,68. Meanwhile, the mean score of pre-test and post-test in the control class was 53,48 and 69,70. There was an improvement from pre-test to post-test as many as 16,23 in the control class. However, the mean score of the experimental class was higher than that of the control class. Furthermore, the t-test result was 2,60 while the t-table with the significant level 0.05 in d.f 66 was 1,66827. The result of t-test was higher than t-table (2,60 > 1,66827). Based on the mean score and t-test calculation, there was a significant improvement after the treatments in the experimental class. In conclusion, the hypothesis was accepted that Group Investigation method was effective to teach writing Announcement text at first grade students at SMK Karya Teknologi Jatilawang in academic year 2015/2016